Electrodynamic signal device



NGV. 13, 192.8. y 1,691,243

c. R. HANNA ELECTRODYNAMIC S IGNAL DEVICE Filed Aug. 25, 1925 WITNESSESZ l INVENTOR V L. '.a

wwmgf Patented Nov. 13, 1928.

UNITED STATES Pn'rEN'r oEElcE.

CLINTON R. HANNA., OF WILKINSBURG, PENNSYLVANIA, ASSIGNOR TO WESTING- HOUSE ELECTRIC & MANUFACTURING COMPANY, A CORPORATION OF PENNSYL- VANIA.

ELECTRORYNAMIC SIGNAL DEVICE.

Application led August 25, 1925. Serial No. 52,268.

My invention relates to signal translating devices, more particularly to a microphone of the electro ynamic type and the circuit with which the microphone is associated.

It is an object of my invention to provide a combination of microphone and circuit of such character that the voltage delivered from the combination shall faithfully represent the sound.

In most devices tor producing, from a sound, an electrical representation of that sound, distortion results because the response ot the device to .the sound is dierent at different frequencies. It is an object of my invention to provide a device inwhich said cause of distortion is largely avoided.

In a micro-phone of Athe elect-rodynamic type, that is, one in which the sound causes a conductor to move in a magnetic field, thus generating an electromotive force, the 'voltage delivered by the microphone is not in proportion to the sound, but varies with the frequency. rlhus, it the sound exert upon the microphone a force ot' constant amplitude but ot increasing frequency, the voltage, instead of being constant, when the amplitude of the force exerted by the sound is constant will change as the frequency increases.

The change in the response of the instrument with change in frequency depends upon the damping, the nature of the forces tending to restore the vibrating system to normal` position, and the inertia of said system. If the microphone is designed to have small damping and small restoring forces, the response will be inversely proportional to the frequency. It is an object of the invention to take advantage of this property of such microphone.

It is an object of my invention to devise a circuit having properties that will counteract the property ot the microphone just described, thereby tending to keep the voltage impressed upon the line by the combination of microphone and circuit proportional to the force exerted by the sound and independent of the frequency.

In the practice of my invention, a circuit is used in which a reactor and a resistor are in series. When the frequency increases, the impedance offered by the reactor will change in one direction, if it be an inductor, and in the other direction, it it be a condenser. It is an object of my invention to take advantage of this change with the frequency to produce a result independent of the frequency.

It is a further object of my invention to provide a microphone of the electrodynamic type having very small internal reactance. By means of such a microphone, itis possible to design a combination of microphone and circuit in which the relation needed to produce an eiicient combination can be obtained without requiring any large inductance in the circuit external to the microphone. The small inductance present in such a circuit can be the'primary of a transformer. This combination, consequently, enables me to use a transformer having a large turn-ratio and thus escapes the necessity that the microphone itself develop a high voltage.

In one form of my invention, the circuit to which the microphone is connected must have small reactance in proportion to its resistance. It is an object of this invent-ion to reduce the reactance of the microphone itself. By such reduction of the reactance, the resistance necessary to make the circuit predominantly resistive can be included in the microphone itself, thus avoiding the use of an external resistance. l

Itis a further object of this invention to provide a short-circuited secondary for the moving coil in the microphone, whereby the effective reactance of such coil is greatly diminished. One means for accomplishing this object is a plating of copper on surfaces adjacent to the air gap in the permanent magnetic circuit.

It has heretofore been proposed to provide a microphone free from abnormal responses to certain frequencies by utilizing the properties of a glow discharge in air. One disadvantage of such microphone is that its properties change with atmospheric conditions. Another disadvantage is that it requires a source of current of very high potential whereby insulation difficulties are produced. My invention produces a microphone which is independent of atmospheric conditions and does not require that the potentials be high.

Other objects "of the invention and structural details will be understood from the following description and the accompanying drawings, in which IFigure l is a diagram of one form of the device,

` The details of this amplifier are not illus Fig. 2 is a diagram of modification, and

Fig. 3 is a section through a portion of the microphone.

The microphone is provided with a diaphragm 1, preferably of the conical type, although the principle herein de scrlbed 1s ap plicable to any microphone 1n Wh1ch the principle reaction is that of inertia. The diaphragm is mechanically connected to a coil 2 which is reciprocated axially by the .nuove-y quencv.

Preferably, the inductance of the coil 6 is equal to the effective inductance of the microphone. In order that the effective inductance of the microphone shall be as small as possible, the surfaces adj acent to the air gap are electroplated, as Shown at 7 and 8, With a material more conductive than the magnets. Each of the rings formed by the plating may be considered a short-circuited secondary, the coil 2 being regarded as a primary. The effect of the short-circuited secondary is to largely neutralize the inductance of the coil 2, producing, in effect, a voltage `generator of greatly diminished internal reactance. I have found that this result may be obtained With a plating of copper only one mil in thickness. Obviously, either one of the two rings may be omitted, if desired.' It is equally obvious that a ,ring of some other form instead of a plating may be employed, when preferred.

The primary 6 of the transformer produces a yoltage in the secondary 10 which is impressed upon the grid 11 of anamplifier.

trated since they are not related to this invention. The output circuit of this amplifier may be directly connected to the line or may be connected through as many stages of amplificationas necessary. At the outer end f of the line I have shown a second amplifier 12 and a transformer 13 by means of which the signalling current is impressed upon the receiver 14. For obtaining the utmost advantage from the invention herein described, the receiver 14 should also be free from abnormal responses at particular frequencies.

In the modification shown in Fig. 2, the microphone is connected directly to the priT mary 6, Without the interposition of a resister. The secondary 10, instead of being connected directly to the grid 11, is connected through a condenser 15. In series With this condenser is a resistor 16, and the grid-cathode circuit of the tube is in shunt to the resistor 16.

The capacity of the condenser 15 is so small that, even at the highest frequencies to which the microphone is subjected, the impedance of the condenser 15 is large, com pared with the resistance of the element 16.

In the microphone, as illustrated in Fig. 3, the dia hragm 1 is supported by a means that oldrs practically no tendency to bring the diaphragm back to a mean position. The support consists of a ring 17, preferably of very flexible material, such as kid leather. Its mechanical effect upon the coil 2 is morcly that of a stop, preventing extreme excursion of the coil but leaving it otherwise free to reciprocate.

In the operation of the device, sound impressed upon the diaphragm 1 reciprocates the coil 2. Voltages are thus developed in the coil 2. The voltages being proportional to t-he velocity of the coil 'will be inversely proportional to the .frequency of the sound. This relation between the fre uency and the voltages holds for any microp one in which the moving parts are not subjected to damping or restoring forces, but present to the force exerted by the sound no resistance except that due to inertia.

- In the circuit shoWnin F ig. 1, as the frequency of the sound increases, the voltage delivered by the microphone diminishes. The current through the" circuit therefore also diminishes, because the impedance of the circuit is nearly constant. The current diminishing, the drop over the non-.inductive element 5 diminishes. I

The inductive element 6 offers a greater impedance at the higher frequency. This increase in impedance tends to counterbalance the effect of the decrease in current, whereby the drop over the inductor 6 is greater than the reduction in current would lead one to expect.

The impedance of the resistor 5 does not change With the frequency. The impedance of the reactor 6 does. The drop through the circuit is, therefore, differently distributed' when the frequency increases, a larger portion of this drop being across the reactor 6. This also tends to counteract the effect of the diminution in current.

The tvvo phenomena, each of which tends to counteract the effect of lessened currentv are together sufficient to completely counteract it, With the result that the potential across the primary 6 is unchanged by a change in the frequency of the sound. The potential across the primary 6 being constant, the p0- tential across the secondary 10 Will also be constant. The output of the amplifier is, consequently, independent of the frequency of the sound. It will vary only With the amplitude of the sound.

neonata ln the form shown in Fig. 2, at increased frequency, a lower potential is delivered from the microphone 2 and, consequently, a lower potential is produced across the coil l0. At the higher frequency, however, the reactance of the condenser l5 is smaller than at low frequency. This reactance is the principal impedance in the circuit 10--16. Diminution of this reactance will, therefore, tend to increase the current and, consequently, will offset the diminution in the voltage impressed upon the circuit. The current through the circuit 10-16 is, therefore, subst. ntially constant. Since the element 16 is non-inductive, thc constant current through it is accompanied hy a constant voltage across it. The voltage between the grid and the cathode of the iirst amplifier tube is, therefore, independent of the frequency.

Many forms of apparatus involving this invention will he apparent to those skilledy in the art. The specific illustration and description of two forms is not to be understood as a limitation. No limitation except that required by the prior art or explicitly indicated in the claim is intended.

I claim as my invention:

In a sound-translating device, a microphone including a coil, means for moving said coil in response to sound, means for producing a magnetic field acrosssaid coil, a cir-k cuit into which said microphone delivers, meansfor minimizing the inductive effect of said microphone in said circuit and a transformer having a primary in said eircuit, the inductance of which is approximately equal to the inductive effect of said coil.

In testimony whereof, I have hereunto suhscribed my name this 14th day of August, 1925.

CLINTON R. HANNA. 

